Flyback transformer device

ABSTRACT

A flyback transformer device wherein a flyback transformer and diode are accommodated in a hermetically sealed metal container which is filled with insulating oil whose amount of charge resulting from corona discharge is less than a predetermined value.

United States Pater [191 Sato et al.

[ 4] FLYBACK TRANSFORMER DEVICE [75] inventors: Kuhei Sato, Ikeda;Shunsaku Shimazu, Shijonawate, both of Japan [73] Assignee: MatsushitaElectric Co., Ltd., Osaka, Japan [22] Filed: Aug. 24, 1971 [21] Appl.No.: 174,417

[30] Foreign Application Priority Data Aug. 27, 1970 Japan 45/75336 Aug.27, 1970 Japan 45/75338 Sept. 11, 1970 Japan 45/90967 [52] US. Cl.336/94 [51] Int. Cl. H0lf 27/02 [58] Field of Search 336/90, 92, 94;321/8 C, 2 HF; 317/14; 315/27 [56] References Cited UNITED STATESPATENTS 4/1933 Corbitt 321/8 C Oct. 16, 1973 3,474,369 10/1969 Keogh336/94 2,478,983 8/1949 Runbaken.... 336/94 3,564,386 2/1971 Leonard321/8 C 3,521,210 7/1970 lwata 336/92 3,546,647 12/1970 Roddy 336/923,234,493 2/1966 Zwelling 336/94 OTHER PUBLICATIONS Ed. .1. B. Birks;Modern Dielectric Materials, 1960, p. 146.

Primary Examiner-J. D. Miller Assistant Examiner-Harvey FendelmanAtt0rneyStevens, Davis, Miller & Mosher [5 7] ABSTRACT A flybacktransformer device wherein a flyback transformer and diode areaccommodated in a hermetically sealed metal container which is filledwith insulating oil whose amount of charge resulting from coronadischarge is less than a predetermined value.

3 Claims, 5 Drawing Figures PAT'ENTEUncT 16 ms ACCELERAUO/V L/FE 77MESHEET 1 0F 5 1 SHOWS NO/VBREAKDOW/V PATENTEDOBI 16 ms 3.766; 505 SHEET30F 3 FIG. 5

1 FLYBACK TRANSFORMER DEVICE vention;

FIG. 2is a graph showing the relationship between the amount of chargeresulting from corona discharge in insulating oil and the accelerationlife time;

FIG. 3'is a sectional view of an apparatus for measuring the quantity ofcharge resulting from corona discharge in the insulating oil;

FIG. 4 is a graph showing the relations of the accelerating 'life timeto the dielectric loss of the insulating oil; and

FIG. 5 is a sectional view showing the flyback transformer according toa second embodiment of this invention.

Referring first to FIG. 1 of the drawings, there is shown the basicconstruction of the flyback transformer device embodying the presentinvention, wherein numeral 1 represents a hermetically sealed metalliccontainer in which is accommodated a transformer 2 which is adapted toproduce a high-frequency high voltage upon being impressed with an inputthrough a low-voltage terminal 3 from outside. The high-frequency highvoltage thus produced is rectified by a high-voltage diode 4 and thentaken out through a high-voltage terminal 5 to be connected with apicture tube 6. The space in the hermetically sealed container isentirely vacuum-impregnated with insulating oil as shown at 7 sothat nogas portion is present within the container.

The flyback transformer 2 comprises a core 2a, primary coil (primarywinding) 2c, and high voltage coil'(secondary winding) 2b, thelow-voltage and highvoltage terminals 3 and 5 including conductors 3aand St: for connecting the inside of the container to the outsiderespectively.

With the flyback transformer device having the aforementionedconstruction, its dielectric strength is greatly increased because ofthe presence of the insulating oil so that the distance for insulationcan be reduced, which makes an effective contribution to miniaturizationof the flyback transformer device. With regard to the stability, safetyand so forth for a long life span, however, serious problems arise whichcannot be solved only by increasing the dielectric strength. The moreimportant of such problems will be explained below.

I. With the oil-insulating system, the distances for insulation betweenthe various parts within the flyback transformer device can be reducedso that the device per se can be effectively miniaturized; however, atthe conductor surfaces to which the high voltage flyback pulse isimparted, it is more likely that corona discharge occurs in the oil, andmoreover the quantity of charge resulting from the corona discharge inthe oil is very large as compared with the case of the conventionalflyback transformer device designed such that the insulation spacebetween the transformer and the metallic shield casing is large enough.With a flyback transformer device of the oil insulating system, it isnecessary to thoroughly inhibit the occurrence of corona discharge inthe oil since the speed of oil deterioration increases with an increasein the quantity of charge resulting from such discharge, whereas with aconventional device of air insulating system, even if a certain amountof corona discharge occurs, air diffusion prevents such discharge fromleading to the destruction of the entire passage, and therefore such aproblem has heretofore been overlooked. In the case of a flybacktransformer device of the oil insulation system, therefore, it isessential that provision be made for means for achieving miniaturizationof the device by restraining deterioration of the insulating materialdue to corona discharge.

2. Due to the fact that insulating oil is contained, if the device isexternally heated or surrounded by flame, there is a possibility thatthe insulating oil is thermally expanded or evaporated so that the oil,either as it is or gassified, leaks and the thus leaking oil or gas isburnt.

3. Since the insulating oil is filled also between the coilsconstituting the flyback transformer and the container, straycapacitacne occurring therebetween is in proportion to the specificinductive capacity of the insulating oil. Moreover, as compared with thecase of a conventional flyback transformer device, the stray capacitanceis increased because of the reduced spacing between the coil mentionedabove and the container. Obviously this will have an adverse effect onthe performance of the flyback transformer device in that the width ofthe flyback pulse is increased, the ringing pulse becomes close to thebasic wave, and so on. For example, there is a tendency for the pictureto be subject to greater distortion and/or disturbance, an increasedamount of heat to be produced in the flyback transformer, and so on.

Concrete methods of solving the aforementioned problems will bedescribed in detail below.

1. Restraint of corona discharge in the oil In a flyback transformerwhich is miniaturized by adopting the oil insulating system, anextremely intense electric field occurs at the surfaces of the highvoltage coil wires to which the high voltage flyback pulse is imparted,lead-out wires thereof and lead-out wires of the high voltage diodesince each of these wires has a small diameter, and therefore there islikelihood that corona discharge occurs there. In order to avoid this,the curvature of those portions to which an intense electric field isimparted may be increased, for example, by using wires of greaterdiameters or by covering the wires with spherical conductors of agreater curvature, so that the electric field may be weakened. From thestandpoint of manufacture of design, however, difficulty is encounteredin an attempt to apply such means to all the portions. (If a coil wirehaving a greater diameter is used for example, the size of the flybacktransformer is increased so that the entire flyback transformer devicebecomes bulky.) Hence, it is difficult to solve the problems describedabove only through the use of such means.

A reasonable and economical construction can be achieved by usinginsulating oil with a high corona discharge starting voltage and coronaresistance in addition to taking the aforementioned countermeasures withrespect to those portions where it is possible. However, thedetermination of corona discharge starting voltage the high voltagecharging portion is imperfect since it depends upon the sensitivity ofmeans for detecting corona discharge, its electrode constructioncondition thereof and so forth. Therefore, the quantity of chargeresulting from corona discharge is measured with such an electrodeconstruction and applied voltage that an easily measurable degree ofcorona discharge occurs, and it has been found that there is a veryclose co-relation between the charge quantity thus measured and theresult of an accelerated life test performed with a flyback transformerdevice using the same kind of insulating oil as in the presentinvention. FIG. 2 is a graph showing the result, from which it will beseen that the life span changes sharply, with a corona discharge chargequantity of about 100 pico coulomb (pC) as the boundary, when a voltageof KVp (flyback pulse) is applied. Referring now to FIG. 3, there isshown an electrode arrangement which is so .designed that the liabilityof occurrence of a corona andthe needle electrode 14 is connected withthe high voltage flyback pulse of 15 KVp through a high voltage lead 15and also with a device for measuring the quantity of charge stemmingfrom corona discharge. The needle electrode is so worked that the pointend thereof has a radius of curvature of am.

In this specific example, measurements were made of the quantity ofcharge stemming from corona discharge when flyback pulse of 15 KVp wasimparted. As a result, it has been found that mineral oil, siliconeliquid and alkylbenzene gave the best results. This can be observed inany flyback transformer device. The reason is that the electric fieldacting on the insulating oil inside the flyback transformer device is anelectric field of a high frequency called a flyback pulse except for aportion thereof and therefore the behavior of the oil is quite differentfrom that of insulating oil commonly used in commercial frequency powerapparatus.

It has been found that if noninflammable insulating oil (diphenylchloride group) which is now extensively used in power transformers orthe like, for example, is employed in the flyback transformer accordingto this invention, deterioration is caused in a shorter time than in thecase where silicone liquid is used as insulating oil. The fact that sucha result is obtained despite the fact that the short-time destructionvoltage (A.C.) stemming from the sphere gap of such noninflammableinsulating oil is 20 30 percent higher than in the case of siliconeliquid, for example, shows the uniqueness of the characteristicsrequired of the flyback transformer device according to this invention.In the case where an electric field of a high frequency such as aflyback pulse is applied, insulating oil under such an electric field iscaused to start corona discharge at a lower electric field than in thecase where a commercial frequency electric field is applied. This is dueto the fact that the speed of deterioration of the insulating oil isaccelerated more than the frequency ratio due to the abnorupon theflyback pulse has a frequency three to five times as high as that of thebasic wave, the frequency of the ringing pulse in the aforementionedcase becomes to 200 KHz. 1

Furthermore, dielectric loss (tan.'6) occurs in an infinitesimal portionof the insulating oil to which a high frequency electric field isapplied, and as it increases, the dielectric loss density (W/cm) in thatportion is increased so that however small the aforementioned portionmay be, this portion of the insulating oil is heated to a hightemperature so as to be subject to thermal decomposition, evaporation,emission of dissolved gas or the like. Thus, aninfinitesimal void isformed in the oil, which becomes a cause for corona discharge. FIG. 4shows the relationship between the dielectric loss of various types ofinsulating oil at 25C and the result of acceleration life testsperformed in a manner similar to that described with reference to FIG.2. From this, it will be noted that it is desirable that the dielectricloss of the insulating oil to be used be less than l X 10 in a frequencyrange of 10 2 X 10 Hz.

2. Noninflammability It is a well known and extensively adoptedtechnique to use noninflammable oil of the diphenol chloride group asinsulating oil with the view to improving apparatus with respect tosafety; however, it has been seen from the foregoing that in the case ofthe flyback transformer according to this invention, it is inappropriateto employ such noninflammable insulating oil. In the flyback transformerdevice according to this invention, there is no possibility that ashort-circuit or a discharge accident tending to lead to fumingorcombustion occurs which sometimes happens in the conventional dry typeflyback transformer device impregnated with wax or the like. However,using mineral oil, alkylbenzene or the like as insulating oil cannot besaid to be the best means, when consideration is given to such casesthat the flyback transformer is externally exposed to flame or excessiveheat due to an accident or fire taking place at the position where atelevision receiver incorporating the flyback transformer device islocated.

Silicone liquid cannot be said to be non-inflammable itself; however, ithas been found that in the case where it is used as insulating oil forthe flyback transformer device according to this invention, siliconeliquid is superior to certain kinds of non-inflammable insulating oil inthat it is very stable, without fuming, with respect to external flameor the like. Thus, by using silicone liquid as insulating oil, thenoninflammability of the flyback transformer device according to thepresent invention is greatly improved.

3. Specific inductive capacity From the standpoint of designing theflyback transformer device, it is obviously desirable that the specificinductive capacity (measured at the frequency of the flyback pulse basicwave or ringing pulse) of the insulating oil adapted for use in thepresent invention be close to unity. In the present invention, however,it is of paramount importance to select insulating oil from thestandpoint of corona discharge. This implies that the flybacktransformer device should be so designed as to represent improved.characteristics whatever value the specific inductive capacity of theinsulating oil in use may have. If the specific inductive capacity ofthe insulating oil to be used has a value up to 4, it is possible toconstruct a flyback transformer device having satisfactorycharacteristics by correspondingly changing the numbers of turns of thecoils, outer diameters of the windings and positional relationshipbetween the primary coil (primary winding) and the high voltage coil(secondary winding), though the situation differs to some extentdepending upon the spacing between the hermetically sealed container andthe coil surfaces. Moreover, in the cases where use is made ofinsulating oil having a specific inductive capacity greater than theaforementioned value, it has been found that readily acceptablecharacteristics can be achieved by increasing the size of the sealedcontainer, the coating thickness of the coil wire and so forth and byconstructing the flyback transformer device as shown in FIG. 5.Description will now be made in connection with FIG. 5, wherein numeralsl to 7 indicate elements corresponding to those shown in FIG. 1respectively, and 8 denotes a spacer molded of a material having a lowspecific inductive capacity such for example as polypropylene or thelike. The spacer 8 is interposed between the sealed container 1 and theflyback transformer 2, serving to reduce stray capacitance which occurstherebetween and decrease the required quantity of the insulating oil tobe used so as to reduce the extent of expansion and contraction of theoil which depends upon the temperature thereof.

With such an arrangement, most of those kinds of insulating oil whichhave extensively been used at the present time can be employed in viewof the operational characteristics of the flyback transformer device.

The advantages of the present invention have already been describedabove, and therefore, in order to avoid repetition, only the major oneswill be mentioned below.

i. The present flyback transformer device can be greatly miniaturized sothat the bulk thereof can be made as small as about one-fifth of that ofthe conventional device.

ii. High safety can be achieved.

iii. High reliability on electric insulation can be achieved, and a longlife span can be secured.

iv. Where silicone liquid is used as insulating oil, by using theaforementioned spacer in combination therewith, it is possible to reducethe required quantity of the expensive silicone liquid. Furthermore,because of the fact that the thermal expansion coefficient of siliconeliquid is about 10 percent higher than those of other kinds ofinsulating oil, it is also possible to decrease the amount ofrespiration which is required of the sealed metallic container to copewith temperature variations occurring in the flyback transformer deviceby reducing the quantity of the silicone liquid to be used.

What is claimed is:

l. A flyback transformer device comprising a hermetically sealedmetallic container; a flyback transformer and a high voltage diodeaccommodated within said hermetically sealed metallic container,electric power being provided to said flyback transformer from outsidesaid container to provide a high DC. voltage output by rectifying thehigh voltage pulses appearing in said flyback transformer; andinsulating oil filling the entire space within said hermetically sealedmetallic container including small gaps therein, said insulating oilbeing characterized by the quantity of charge stemming from a coronadischarge occurring between a needle electrode and a flat electrode (thepoint end of said needle electrode has a radius of curvature of 20 um,and the spacing between said two electrodes is 5 mm) immersed in saidoil being pico coulomb (pC) ofless when a flyback pulse of 15 KVp isapplied to said electrode.

2. A flyback transformer device according to claim 1, wherein use ismade of insulating oil of which the dielectric loss is less than 1 X 10at 25C in a frequency range of 10" to 2 X 10 Hz.

3. A flyback transformer device according to claim 1, wherein saidinsulating oil, use is made of silicone liquid.

1. A flyback transformer device comprising a hermetically sealedmetallic container; a flyback transformer and a high voltage diodeaccommodated within said hermetically sealed metallic container,electric power being provided to said flyback transformer from outsidesaid container to provide a high D.C. voltage output by rectifying thehigh voltage pulses appearing in said flyback transformer; andinsulating oil filling the entire space within said hermetically sealedmetallic container including small gaps therein, said insulating oilbeing characterized by the quantity of charge stemming from a coronadischarge occurring between a needle electrode and a flat electrode (thepoint end of said needle electrode has a radius of curvature of 20 Mu m,and the spacing between said two electrodes is 5 mm) immersed in saidoil being 100 pico coulomb (pC) of less when a flyback pulse of 15 KVpis applied to said electrode.
 2. A flyback transformer device accordingto claim 1, wherein use is made of insulating oil of which thedielectric loss is less than 1 X 10 1 at 25*C in a frequency range of104 to 2 X 105 Hz.
 3. A flyback transformer device according to claim 1,wherein said insulating oil, use is made of silicone liquid.